Nonvolatile memory devices are widely used in computers, memory cards, and other electronic devices because they can retain data after removal of a power supply. Flash memory is one type of nonvolatile memory device.
Flash memory can be categorized as a NOR or NAND memory type device based on how its memory cells are connected to bit lines. In a NOR type flash memory, two or more memory cell transistors are connected in parallel to one bit line. The NOR type flash memory can store data by a channel hot electrode method, and can erase data through Fowler-Nordheim tunneling (F-N tunneling). In a NAND type flash memory, two or more memory cell transistors are connected in serial to one bit line. The NAND type flash memory stores and erases data through F-N tunneling.
NOR type flash memories can be less suitable for use in high integration density devices due to their larger dissipation of current than NAND type flash memories. Consequently. NAND type flash memories may be more easily integrated into higher density devices.
A cell array in a NOR type flash memory can include a plurality of banks, where each of the banks includes a plurality of sectors, and each of the sectors includes a plurality of memory cells. An erase operation of a NOR type flash memory may be carried out in groups of sectors, and a programming operation may be performed in groups of words (or bytes).
To program data into the cell array of a NOR type flash memory, a program command is issued to the flash memory. Then, addresses and data to be programmed are provided to the flash memory. The addresses and data are temporarily stored in the memory chip, and memory cells that corresponds to the program addresses are selected. A voltage that correspond to the program data is applied to bit lines that correspond to the selected memory cells, which programs the data into the memory cells of the memory device. When a predetermined program operation execution time elapses, a verify operation is performed, which determines whether each of the data is correctly programmed in a selected memory cell. The program and verify operations are repeated until the data is correctly programmed in the selected cell.
With the channel hot electrode method of programming a NOR type flash memory, a high voltage of about 4V to 6V can be applied to a drain of the memory cell. The high voltage that is applied to the drain is usually generated through a charge pump within the memory chip, which can limit the number of memory cells (i.e., memory bits) that can be simultaneously programmed to, for example, 2 to 4 memory cells. When the number of memory cells that can simultaneously programmed is limited to four, then 16-bit data can be divided into four groups of four bits, and programmed into the memory cells through four sequential programming operations.
Before a flash memory is programmed, a portion of addressed memory cells should be erased by setting it to a logic “1” value. Memory cells are then programmed by either leaving their logic value as “1”, or by switching their logic value to “0”.